1. Field of the Invention
The present invention relates in general to an apparatus for accurately positioning a work piece in relation to a working tool, and more particularly to a miter guide for table saws or other cutting or shaping tools, such as those used in woodworking.
2. Description of the Related Art
Typical miter guides currently in use are comprised of a calibrated protractor fence which is pivotally attached to a steel bar. The protractor fence is usually an aluminum casting approximately 6 inches wide and 13/4 inches high. There is a blind pivot pin centered in its width attaching the casting to the guide bar and allowing the fence to rotate equally 45 degrees left and 45 degrees right, thereby changing the relative angle between the work piece and the cutting blade. Projecting vertically from the radiused protractor portion of the casting, there is a handle threaded into the steel guide bar that can be loosened to allow rotation of the fence and tightened to secure the fence in the desired angular orientation. Once the desired angle is selected and secured, the work piece is held tightly against the face of the casting. The casting is then manually moved forward in a straight line as the guide bar slides in the slot in the table top, thereby feeding the work piece into the plane of the cutting blade at the relative angle necessary to produce the desired cut. This configuration is universally typical in standard equipment miter guides which are included with the purchase of new woodworking table saws.
Some practical deficiencies in this approach to crosscutting a work piece are as follows. The surface area of the fence is proportionately too small to effectively stabilize the material being cut, consequently the material must be held very tightly against the face of the guide to counter the vibration that occurs when the work piece comes into contact with the rotating blade. Ultimately, this vibration causes slight lateral movement in the work piece, across the face of the guide, resulting in a finished cut that is not flat or true. This lack of fixed stability in the work piece can also increase the potential for the accidental kick-back of the work piece.
The danger of kick-back and potential for inconsistent cuts is so prevalent that many woodworkers modify the standard miter guide by attaching a longer, wider wooden face to the casting, thereby increasing the surface area against which the work piece can be secured. This is only moderately effective in resolving the problems. While the resulting increase in the surface area of the fence helps reduce the lateral movement caused by vibration, it also effectively creates a lever that works against the locking mechanism. The distance between the pivot point and the center of the tightening handle is often less than 3 inches. An extended face of only 12 or 14 inches can multiply the force against the locking handle and allow the gauge to slip. In addition, pushing or pulling the outermost end of the extension will invariably induce flexing, because the extension is only secured to the face of the casting. The outer end of the added piece is free to move as much as the material it is made of will allow. That is, even with the handle securely tightened, the outermost end of the wooden extension can flex sufficiently to compromise the integrity of the angle and of the face of the cut.
Still another deficiency in the current art is the method by which the desired angle is selected and secured. As the guide is rotated, a calibrated scale on the top face of the radiused portion of the casting passes beneath a stationary pointer. The increments (degrees) are indicated by raised or recessed lines. These lines are often foundry cast. The lines can be wide enough for the casting to move more than one half degree without taking the stationary pointer off the line, thereby compromising the accuracy of the setting.
In Macksound (U.S. Pat. No. 4,881,437), a longer fence is provided, and the outside end is stabilized with a diagonal bar. However, this design is self defeating due to the method of achieving variable angles. A three point configuration is used in an attempt to establish the desired stability inherent in a broad based triangle. However, as the fence is adjusted outward to a more acute angle, the movable arm that slides through the slot in the fence must move progressively closer to the plane of the cutting blade. When the device is set up to make a 45 degree cut, the three point base becomes very narrow. The outermost edge of the fence is no longer directly supported by the diagonal bar. This is the functional equivalent of a typical miter guide with an extension attached to the face. The outer end is free to flex and acts as a lever working against the locking mechanism that secures the device at a given angle.
In Frame (U.S. Pat. No. 4,494,429), an extended fence is added to the existing protractor type miter guide, and the outer end is stabilized with a stabilizing bar. Again, the intention was apparently to gain the benefit of the longer guide face and eliminate the problem of instability created by doing so. This approach is more effective than that of the previous example at keeping the end of the fence stationary. However, it is more elaborate and requires significant setup before it can be used. The process of changing angles for each different cut requires removing and replacing pins that fit into preset holes for specific angles. Additionally, when the device is set up to cut there are two guides that must move simultaneously to ensure accuracy. The mechanism has several links and adjustable contact points that would make it difficult to advance the entire assembly forward without one guide dragging or binding. It is not a device that is quick and easy to use.
The device disclosed by Hallenbeck (U.S. Pat. No. 5,016,508) maintains a wide based triangular configuration at any of the angles within its limited range. This ensures that at any setting the long fence is stable at both ends, and it significantly reduces the potential for deviation from the desired angular cut due to leverage against the locking mechanism or flexing. However, this device is cumbersome and involved. It requires a sliding table that covers the top of the saw. Its range of adjustment is limited. The fence starts out at a 90 degree cut and advances forwardly presumably to 45 degrees plus. There is no provision, however, for drawing the fence back to make cuts in the opposite direction. Any opposing cuts must be laid out on the back of the work piece and cut upside down. The main pivot at the end of the fence closest to the plane of the cutting blade is so far back toward the operator, that by the time the work piece has been fed completely through the cutting plane, the entire assembly is practically hanging off the far end of the table saw. Knobs and pins must be removed and replaced to change angles. With so many adjustments in different parts of the assembly, a little slop in each one cumulatively creates the potential for significant inaccuracy, in addition to the inconvenience of the set up process.
Each of the above described devices only work on one side of the saw blade and cannot be set up to feed from the other side. This is a disadvantage because the blade on some table saws adjusts angularly from 90 degrees (straight up) to 45 degrees left. Others adjust from 90 degrees to 45 degrees right. These designs are complicated to set up and limited in their usefulness.